The present invention relates to an optical disk used in a filing system or audio video system.
One type of optical disk, the air sandwich type, has a hollow portion formed between a protective plate for protecting a recording layer and a substrate for supporting the protective plate. In an optical disk of this type, a laser beam must be transmitted through the protective plate with high precision, resulting in that a thickness and a material of the protective plate are thereby limited.
Conventionally, in an optical disk of this type having a plastic protective plate, the protective plate is not rigid so that it bends or warps due to changes in temperature, humidity or its own weight. In order to prevent disk distortion, a conventional optical disk 2 is known, as shown in FIG. 1. The optical disk 2 comprises a protective plate 6 made of a light-transmitting material and a substrate 8 made of a material more rigid than that of the protective plate 6. A recording layer 4 is formed on the inner surface of the protective plate 6. The protective plate 6 is formed integrally with the substrate 8 through an adhesive 9 such that a space 7 is formed between the protective plate 6 and the substrate 8.
Thus, in the optical disk 2, the material of the protective plate 6 differs from that of the substrate 8. When the protective plate 6 and the substrate 8 expand or contract due to a change in temperature or humidity, their expansion or contraction ratios differ from each other, thereby deforming the optical disk as a whole. In particular, in an air sandwich type optical disk, since the peripheral portion between the substrate 8 and the protective plate 6 must be sealed by the adhesive 9 in order to prevent dust from entering, a warp is formed when the protective plate expands relative to the substrate 8, as shown in FIG. 2. However, when the protective plate 6 contracts relative to the substrate 8, an inverted warp is formed, as shown in FIG. 3. When the protective plate 6 expands in the case where the substrate 8 is more rigid than the protective plate 6, a wave is formed in the protective plate 6, as shown in FIG. 4. However, when the protective plate 6 contracts in this case, the protective plate 6 separates from the adhesive 9 or is damaged, as shown in FIG. 5. As a result, the optical disk itself is damaged.
The warp and wave of the protective plate cause error in the incident angle and the optical path length of a laser beam, or degradation of data processing precision of the optical disk due to dust or the like entered from the portion that has separated. In the worst case, data processing cannot be performed.